1. Field of the Invention
The present invention relates to an optical waveguide device used for optical information processing and optical measurement control performed utilizing a coherent light source, and a light source device and an optical apparatus including the optical waveguide device.
2. Description of the Related Art:
In the fields of optical information recording and reproduction, a higher density of recording and reproduction is realized by using a light source for emitting light having a shorter wavelength. For example, whereas conventional compact disk apparatuses use near infrared light having a wavelength of about 780 nm, digital versatile disk (DVD) apparatuses for recording and reproducing information at a higher density use red semiconductor laser light having a wavelength of about 650 nm. In order to realize a next-generation optical disk apparatus for recording and reproducing information at a still higher density, development of blue laser light source devices have been actively developed. For example, a wavelength conversion element using a non-linear optical substance has been developed to be included in a compact and stable blue laser light source device.
FIG. 14 is a schematic view illustrating an exemplary blue light source device using a second harmonic generation element (hereinafter, referred to as an "SHG element") 117. With reference to FIG. 14, the SHG element 117 will be described.
The SHG element 117 includes a dielectric substrate 114 and a high refractive index area having a width of about 3 .mu.m and a depth of about 2 .mu.m formed by a proton exchange method. The high refractive index area acts as an optical waveguide 115. Infrared light emitted from a semiconductor laser 111 having a wavelength of about 850 nm is collected on an incident surface 139 of the SHG element 117 through a collection lens 112 and then propagated through the optical waveguide 115 in the SHG element 117 to form a fundamental guided wave.
Lithium niobate crystals forming the dielectric substrate 114 have a non-linear optical constant. As a result of a sufficiently large non-linear optical constant, a harmonic guided wave having a wavelength of about 425 nm is obtained by wavelength conversion of the infrared light, and excited from the electric field of the fundamental guided wave.
In order to compensate for a propagation constant difference between the fundamental guided wave and the harmonic guided wave, domain inversion areas 116 are periodically formed in the optical waveguide 115. The harmonic guided waves which are excited throughout the optical waveguide 115 are coherently added together and then come out from an outgoing surface 138 of the SHG element 117.
In order to correctly compensate for the propagation constant difference between the fundamental guided wave and the harmonic guided wave, the wavelength of the fundamental guided wave needs to be maintained at a certain value. Accordingly, as the semiconductor laser 111, a DBR laser is used for its very small wavelength fluctuation in accordance with the temperature or the like. A DBR laser has another feature in that since light is oscillated at a single wavelength, the light has a satisfactorily high coherency and a satisfactorily low RIN (relative intensity noise).
FIG. 15 is a schematic view of an optical disk pickup including the SHG element 117 shown in FIG. 14 for providing blue light. With reference to FIG. 15, an operation of the optical disk pickup will be described.
Harmonic blue light output by the SHG element 117 passes through a collimator lens 113, a polarization beam splitter 120, a 1/4 wave plate 121 and an objective lens 122 and then is collected to an optical disk 124.
The light modulated by the optical disk 124 is reflected by the polarization beam splitter 120 and guided to a light detector 125 by a collection lens 123. Thus, a reproduction signal is obtained.
The SHG element 117 outputs linearly polarized light in a direction parallel to the page. This light passes through the 1/4 wave plate 121 and returns through the 1/4 wave plate 121 to become a polarized light which is in a direction perpendicular to the page. Thus, the light reflected by the optical disk 124 is all reflected by the polarization beam splitter 120 and does not return toward the SHG element 117 theoretically.
However, the optical disk 124 includes a material having a birefringence. Accordingly, in actuality, an unnecessary polarized component returns toward the SHG element 117 through the polarization beam splitter 120.
While data stored in the optical disk 124 is reproduced, the objective lens 122 is positionally controlled to focus the light accurately to the optical disk 124. Accordingly, the outgoing surface 138 of the SHG element 117 and the optical disk 124 form a confocal optical system, Thus, the light reflected by the optical disk 124 is accurately collected at the optical waveguide 115 on the outgoing surface 138 of the SHG element 117.
In an optical system including a semiconductor laser as a light source, the light component which returns toward a light source after being reflected induces noise (mode hop noise). Conventionally, various proposals have been made for avoiding the mode hop noise.
For example, oscillation in a plurality of longitudinal modes is caused by modulating light from the semiconductor laser with a harmonic signal or by causing self-oscillation of the semiconductor laser.
In the field of optical communication, for collecting light from a semiconductor laser to an optical fiber, a light isolator utilizing a magneto-optical effect is commonly inserted between the semiconductor laser and the optical fiber.
Japanese Laid-Open Publication No. 5-323404 discloses a method, by which an incident surface of an optical fiber or an optical waveguide is obliquely polished, so that the returning light is obliquely reflected and does not return to the semiconductor laser.
These technologies are for reducing the mode hop noise induced by the light returning to inside the semiconductor laser as a light source.
The present inventors performed experiments on data reproduction by an optical pickup including the SHG element 117 shown in FIG. 15. As a result, the present inventors found a noise which is generated by the following mechanism, which is different from induction by the returning light.
The returning light collected at the optical waveguide 115 on the outgoing surface 138 of the SHG element 117 is reflected by the outgoing surface 138 and interferes with the light coming out from the optical waveguide 115. Thus, an interference noise is generated.
Due to such an interference noise, the optical power output from the SHG element 117 appears to have been changed from the optical disk 124, and thus a reproduction signal from the optical disk 124 is modulated with a low frequency noise, resulting in signal deterioration.
Whereas noise induced by the returning light is generated by the interaction of the light inside the semiconductor laser 111 and the returning light reflected by the incident surface 139 of the SHG element 117, the interference noise is generated by the interference of the light from the SHG element 117 and the returning light reflected by the outgoing surface 138 of the SHG element 117.
The present inventors found another cause of the interference noise as a result of a further research. A portion of the returning light from an external optical system external to the optical waveguide device including, for example, collimator lens 113) is re-excited in the optical waveguide 115 as a guided wave and reflected by the incident surface 139 of the SHG element 117. The light reflected by the incident surface 139 is interfered with the light from the semiconductor laser 111. Such an interference also causes the interference noise.
As described above, an optical system including an optical waveguide device involves two different types of noises One is a low frequency interference noise caused by the Interference, in an external optical system, of (1) light emitted by the light source and propagating through the optical waveguide device toward the external optical system and (2) the light reflected by an outgoing surface or an incident surface of the optical waveguide device after propagating through the optical waveguide device and being reflected by an external object (e.g., the optical disk). The other is the mode hop noise caused inside the semiconductor laser.
Various proposals have been made in order to reduce the mode hop noise, but the interference noise caused in the external optical system has not been a target of attention and no proposals have been made for solving the problem of the Interference noise.